Curculigo latifolia is a plant spontaneously growing in west Malaysia and a southern part of Thailand, which is classified into Liliaceae. It is said that the curculin isoform (referred to as curculin hereinafter) contained in the plant is useful as a taste-modifying substance giving sweet taste when the isoform is eaten before drinking water or eating a sour substance.
Conventionally known subunits constituting curculin include for example curculin A and curculin B.
The entire amino acid sequence of curculin A has been determined (see for example patent literature 1). The entire amino acid sequence of curculin B and the nucleotide sequence thereof are disclosed. It is verified that curculin B is different in the amino acid composition from curculin A in terms of several amino acids (see for example patent literature 2).
It has been understood that these subunits both constitute curculin in a form of homodimer and have a taste-modifying function. However, the taste-modifying functions thereof are not sufficient enough to be added into foods.
In accordance with the invention, however, it was succeeded to identify a novel dimeric protein with a heterodimer structure comprising two subunits having different amino acid sequences, namely NAS and NBS. The protein is designated as neoculin. The results are disclosed (see non-patent literature 1). It is first identified in the invention that neoculin has a taste-modifying function sufficient enough for addition into foods.
So as to enable the highly efficient production of neoculin, the results of the expression of the genes encoding the subunits with two different amino acid sequences, namely NAS and NBS, in a prokaryote Escherichia coli are disclosed (see for example non-patent literature 2).
Possibly due to the generation of NAS and NBS in the form of inclusion body in the bacterial cell of Escherichia coli, an accurate heterodimer of NAS and NBS with the exertion ability of the taste-modifying activity can scarcely be generated via the simple expression in Escherichia coli. 
So as to exert the taste-modifying function, then, procedures have been required, including recovery of the inclusion body, solubilization thereof using a solubilizing agent solution such as guanidine chloride, and reconstitution. The requirement of the use of such reagent is disadvantageous in terms of laborious works and production cost. Additionally, such use of the reagent is not suitable for industrial production from the standpoint of safety profile when the produced neoculin is used for foods.
In such circumstances, it has been desired to screen for a substance with higher practical applicability and a better taste-modifying function and to develop a safe and efficient production method applicable to foods
Patent literature 1: JP-A-Hei 3-190899
Patent literature 2: JP-A-Hei 6-189771
Non-patent literature 1: “Biosci. Biotechnol. Biochem.”, Vol. 68, No.6, p. 1403-1407, 2004
Non-patent literature 2: “FEES Letters”, Vol. 573, p 135-138, 2004